The present invention relates to an assembly and method of positioning a workpiece, such as pipe, tubing or any other part that, when clamped, does not line up to the true centerline of a self-centering power chuck due to imperfections of roundness, wall thickness and straightness. The present invention will move the imperfect part""s average centerline to the true centerline of the self-centering power chuck. The present invention uses at least two chuck jaw assemblies, each of which is adjustable by means of an independent internal hydraulic system.
When a workpiece, such as pipe, tubing or another part to be machined (hereinafter simply referred to as xe2x80x9cpipexe2x80x9d) is clamped in a machine tool lathe for threading, it is usually held in place with a self-centering power chuck on the front and rear of the lathe, and the remaining length of the pipe or tubing is supported by rollers. Pieces of pipe or tubing, which come in various sizes and lengths, are not perfectly round or straight from end to end. These imperfections cause the surface area to be machined and threaded to run out from the true centerline of the self-centering power chuck.
Presently, the primary method of correcting the pipe""s centerline runout is to insert shims between the chuck laws and the pipe or tubing. When a shim is used, the chuck must be opened allowing the pipe to move from the clamped position, and a shim must be placed in the correct location to adjust the part""s average centerline with respect to the centerline of the power chuck. The shim must be held in place while the chuck is closed. This process may have to be performed repeatedly until the pipe is at an acceptable location. The process is dangerous and time consuming. In addition, the shim can also reduce the holding properties of the chuck jaw""s gripping surface, which could cause the pipe to slip or move during the machining process.
Other methods for correcting the runout include the use of specialized chucks such as a self-compensating chuck or a sequencing chuck. In both cases, the machine operator has less control of the pipe""s location. A self-compensating chuck, which relies on an external locating device to hold the pipe in place before the pipe is clamped by the chuck, does not always locate the pipe in an acceptable location due to the various imperfections of the pipe. Also, the weight of the pipe has some negative effects on the external locating device. A sequencing chuck has a locating device built into the chuck that retracts into the chuck body after the part is located and clamped. This chuck is very expensive and also has the same locating problems with the various imperfections of the pipe described above. In addition, when these specialized chucks are used the jaws and the locating device must be changed for each size pipe to be machined, increasing the time and cost to set up the specialized chucks.
None of the present methods provides an economical method of locating a pipe for machining.
The present invention overcomes the problems present in the prior art by providing a means for clamping a pipe, then adjusting the imperfect pipe""s average centerline by moving one or more of the chuck jaw assemblies by activating its independent, closed hydraulic system in order to adjust the length of the chuck jaw assembly. As the length of a chuck jaw assembly is reduced, the imperfect pipe""s average centerline moves toward that chuck jaw assembly and all the other chuck jaws follow the pipe due to the dynamic clamping properties of a power chuck. On a power chuck with two or more jaws, the imperfect pipe""s average centerline can be moved to any location within the combined adjustable range of all the jaw assemblies.
The jaw assemblies of the present invention can be mounted onto the power chuck""s master jaw or can incorporate the master jaw as combined assemblies.
Each jaw assembly consists of a base jaw that is affixed to the chuck, a connecting block that moves when the hydraulic system is actuated, and an interchangeable swivel insert with a serrated gripping surface. The swivel insert, which is sized according to the diameter of the pipe being machined, compensates for the roundness imperfections of the pipe and reduces the distortion of the pipe caused by the clamping pressure of the chuck by equally distributing the forces across the entire gripping surface of the swivel insert. Each of the swivel inserts for the jaw assemblies is attached to the connecting block with a single bolt so that it can be quickly changed, depending on the diameter of the pipe being machined.
To activate the invention, the chuck must be clamped on the pipe to be machined. The clamping force of the chuck pressurizes the hydraulic system in each jaw assembly. The hydraulic system consists of a single action cylinder, which is built into the connecting block; a piston, which is affixed to the base jaw; a diverting ball valve assembly; two reservoirs; and a check valve assembly. In the unclamped position, the cylinder is completely extended and full of hydraulic oil. In the clamped position, the hydraulic oil in the cylinder is pressurized. With the hydraulic oil under pressure in the cylinder, a fixed amount of the hydraulic oil is removed from the cylinder by the diverting ball valve. The diverting ball valve has three ports: one port to the cylinder; one port to a spring-loaded fixed area reservoir, which is always open to the center of the ball passage, and one port to a spring-loaded hydraulic oil holding reservoir. When the ball valve is turned, opening the passage from the cylinder through the ball, a fixed amount of hydraulic oil fills the spring-loaded fixed area reservoir, reducing the distance between the base jaw and the swivel insert. The measure of the distance reduced is set by the volume of hydraulic oil held in the spring loaded fixed area reservoir, which is adjustable. The resulting reduction in length of the jaw assembly moves the pipe clamped by the chuck jaws toward that particular jaw. When the ball valve is turned to open the passage to the spring-loaded hydraulic oil holding reservoir, since the minimum spring pressure of the spring-loaded fixed area reservoir is greater than the maximum spring pressure of the spring-loaded hydraulic oil holding reservoir, all of the hydraulic oil from the fixed area reservoir is forced into the hydraulic oil holding reservoir, ending one adjustment cycle. Each jaw assembly can be adjusted in the same fashion in order to move the clamped pipe to any location within the adjustable range of the combined jaw assemblies. The number of cycles which one jaw assembly can be adjusted depends on the volume of hydraulic oil in the cylinder.
When the pipe is unclamped from the chuck, the spring pressure of the spring loaded hydraulic oil holding reservoir forces all the hydraulic oil back to the cylinder through a check valve. This action restores the original length of each of the jaw assemblies for the next pipe to be clamped.
The jaw assemblies could also be activated by an external mechanical device controlled by a machine tools computer program, resulting in a completely automated process. In that event a spool valve rather than a diverting ball valve would be used.